Answer:
The correct answer is: C. Saben.
¿Saben ustedes para qué sirve el documento?
Explanation:
First of all, as the subject of the sentence is USTEDES, the only possible options would be CONOCEN and SABEN.
However, the question here is: "What is the difference between saber and conocer?"
We use conocer to confirm or deny the familiarity with a person, thing, place, etc.
¿Conoces a mi tía Marta? (meaning "Have you met my aunt Marta?")
¿Conoces esta canción? (meaning "Have you heard?" or "Does it sound familiar?")
Conozco muy bien este barrio. (meaning "I am familiar with this neighborhood.")
On the other hand, we use saber to say that we KNOW something (a fact, information, skill, etc.). It normally means that we can explain something, repeat it, do it, etc.
¿Sabes francés? ("Do you speak French?" meaning that it is something you can do)
Sé contar en alemán. (I can count in German.)
Me sé está canción de memoria. (I know this song by heart.)
So, in the example above, the question ¿Saben ustedes para qué sirve este documento? means Do you KNOW...?, and it supposes that you would be able to explain it.
